Sorption is the predominant process governing metal ion movement in soils and includes the following physical and chemical mechanisms: adsorption, precipitation and absorption. The research attempts to use the Hard-Soft Acid-Base Principle to explain sorption selectivity of the metal cations Cd, Ni and Zn by kaolinite and montmorillonite clays. The hard-soft character of the clay surfaces, which is due to their surface functional groups, may be inferred by cation sorption selectivity experiments where pH and complex ion formation are controlled and monitored. Calcium saturated clays were suspended in Ca(Cl04)2, CaCl2 or CaSO4 and spikes of the above metals as divalent cations were added and their reactivity assessed within the framework of the HSAB Principle. For kaolinite in Ca(ClO4)2, metal sorption followed the sequence Cd > Zn > Ni. For montmorillonite in Ca(ClO4)2, metal sorption followed the sequence Cd approximately = Zn > Ni. In CaCl2 and CaSO4, the selectivity was different due to the presence of Cl(-) and SO4(2-) ions which competed with the mineral surfaces for the divalent metal cations. (Copyright (c) 1988, Soil Society of America.)